Road vehicles, especially omnibuses, are used for a variety of different types of operations, which may be conveniently categorised as follows:
(a) central business district (CBD) or local school operations, typically travelling up to 100 km/day; PA1 (b) commercial non-transit operations, typically travelling around 120 km/day; PA1 (c) suburban transit operations, typically travelling from 100 to 200 km/day; and PA1 (d) long distance transit operations, typically travelling 400 km or more each day. PA1 (a) a power splitting mechanical transmission for coupling to a final drive of the vehicle; PA1 (b) a first drive unit arranged for regenerative operation and coupled to the power splitting mechanical transmission; PA1 (c) a second drive unit arranged for regenerative operation and coupled, independently of said first drive unit, to the power splitting mechanical transmission; PA1 (d) a non-regenerative third drive unit arranged in parallel to said power splitting mechanical transmission, to the final drive; and PA1 (e) propulsion control means, for coordinating operation of the drive units in accordance with a plurality of predetermined modes corresponding to a driving cycle of said vehicle; wherein each of the first, second and third drive units includes a different type of energy storage means. PA1 (a) the ring gear of the epicyclic gearbox is arranged for coupling to the tailshaft; PA1 (b) the first drive unit is coupled to the sun gear of the epicyclic gearbox; PA1 (c) the second drive unit is coupled to the planetary gear carrier assembly of the epicyclic gearbox; and PA1 (d) the third drive unit is coupled to the tailshaft via a speed changing transmission; whereby the epicyclic gearbox delivers torque to the tailshaft for propelling the vehicle or receives torque from the tailshaft for retarding the vehicle. PA1 the first drive unit includes an electrical energy storage means and an associated electrical energy conversion means; PA1 the second drive unit includes a fluid pressure storage means and an associated fluid pressure energy conversion means; and PA1 the third drive unit includes a chemical energy storage means and an associated combustion engine. PA1 (i) an acceleration mode, wherein the fluid pressure energy conversion means or the electrical energy conversion means supplies power to the tailshaft, supplemented by power from the combustion engine as required; PA1 (ii) a cruise mode, wherein the electrical energy conversion means supplies power to the tailshaft, supplemented by power from the combustion engine as required; PA1 (iii) a deceleration mode, wherein each of the fluid pressure energy conversion means and the electrical energy conversion means are operated regeneratively to recover power from the tailshaft and supply, respectively, the fluid pressure storage means and the electrical energy storage means; or PA1 (iv) a stationary mode, wherein a selected drive unit supplies power for replenishing the fluid pressure storage means and/or the electrical energy storage means as required. PA1 (a) the planetary gear carrier assembly of the epicyclic gearbox is arranged for coupling to the tailshaft; PA1 (b) the first drive unit is coupled to the ring gear of the epicyclic gearbox; PA1 (c) the second drive unit is coupled to the sun gear of the epicyclic gearbox; and PA1 (d) the third drive unit is coupled to the tailshaft via a speed changing transmission; whereby the epicyclic gearbox delivers torque to the tailshaft for propelling the vehicle or receives torque from the tailshaft for retarding the vehicle. PA1 the first drive unit includes a fluid pressure storage means and an associated fluid pressure energy conversion means; PA1 the second drive unit includes a mechanical energy storage means; and PA1 the third drive unit includes a chemical energy storage means and an associated combustion engine. PA1 (i) an acceleration mode, wherein the mechanical energy storage means supplies power to the tailshaft, controlled by power either supplied to or sourced from the fluid pressure energy conversion means and supplemented by power from the combustion engine as required; PA1 (ii) a cruise mode, wherein the combustion engine supplies power to the tailshaft, supplemented by power from the mechanical energy storage means controlled by the fluid pressure energy conversion means as required; PA1 (iii) a deceleration mode, wherein fluid pressure energy conversion means is operated to control regeneration of power from the tailshaft to supply either the mechanical energy storage means or the fluid pressure storage means; and PA1 (iv) a stationary mode, wherein a selected drive unit (normally the combustion engine) supplies power for replenishing the mechanical energy storage means and/or the fluid pressure storage means as required. PA1 the first drive unit includes an electrical energy storage means and an associated electrical energy conversion means; PA1 the second drive unit includes a mechanical energy storage means; and PA1 the third drive unit includes a chemical energy storage means and an associated combustion engine. PA1 (i) an acceleration mode, wherein the mechanical energy storage means supplies power to the tailshaft, controlled by power either supplied to or sourced from the electrical energy conversion means and supplemented by power from the combustion engine as required; PA1 (ii) a cruise mode, wherein the combustion engine supplies power to the tailshaft, supplemented by power from the mechanical energy storage means controlled by the electrical energy conversion means as required; PA1 (iii) a deceleration mode, wherein electrical energy conversion means is operated to control regeneration of power from the tailshaft to supply either the electrical energy storage means or the mechanical energy storage means; and PA1 (iv) a stationary mode, wherein a selected drive unit (normally the combustion engine) supplies power for replenishing the mechanical energy storage means and/or the electrical energy storage means as required. PA1 determining current state of the propulsion system by monitoring status of each drive unit, including respective operating speeds and energy storage levels; PA1 receiving a demand signal indicative of desired vehicle motion; and PA1 if the demand signal indicates that negative wheel power for braking the vehicle is desired: PA1 if the demand signal indicates that positive wheel power for cruise or acceleration is desired: PA1 if the demand signal indicates that no wheel power is desired, the vehicle being stationary:
Table 1 (overpage) sets out some typical operating parameters for each of these categories. The parameters include the average number of stops likely to be made by the omnibus per kilometre, the hours of operation per day, the opportunities available to replenish the battery energy source, if used, the relative requirement for smooth vehicle operations, the relative importance of energy regeneration and transmission efficiency, and the number of passenger seats. It will be appreciated from the following parameters, that a wide variety of road load environments are encountered during omnibus operations. Such road load environments call for significant flexibility in specifying propulsion systems for these vehicles.
For the purposes of the specification, categories (a) and (b) will be hereinafter collectively referred to as "non-transit" operations, whilst categories (c) and (d) will be hereinafter collectively referred to as "transit" operations. Conventional omnibuses are generally propelled by a relatively high powered compression ignition engine fuelled by diesel. In some cases, typically for non-transit operations, omnibuses may be propelled by electric motors supplied from storage batteries.
TABLE 1 Typical parameters for vehicle operating categories Commer- cial Transit Transit CBD non-transit School Short haul Long haul Average 12-15 15-20 20 12-20 25-35 speed (km/h) Distance 100 120 2 .times. 35 120-200 400 (km/day) Stops/km 3-4 3 2-3 2 1-2 Hours/day 8 14 2 .times. 2 10 16 Daytime battery charging - * Opportu- No Several One No No nity * Fast Yes Yes No Yes No Smooth Essential Important Not Important Not operation Essential Essential Regenerative Very Imp. Important Very Important Not Important Important Transmission Very Very Important Important Important efficiency Important Important Seats 30-45 20-30 45-50 30-45 45
The problems and drawbacks with these propulsion units include, in the case of compression ignition engines, high noise levels, environmental pollution and high fuel consumption resulting from operating at part load or idle for long periods. Omnibuses for transit operations are traditionally powered by diesel engines with power outputs in the range of 140 to 185 kW and typically the engine and transmission have a combined mass of 500 to 800 kg. The engine is usually coupled to an automatic transmission with 4 or 5 gears, with more recent variants including a lock-up torque converter in the top two gears. A half loaded 12 metre omnibus travelling at 60 km/h has approximately 1.8 MJ of energy, this is equivalent to 100 kW of continuous power available for absorption during a relatively slow stop of 18 seconds and 150 kW for a more usual stopping time of 12 seconds. This energy, which might otherwise be recovered, is merely dissipated through friction braking and/or engine retardation braking in prior art propulsion systems. The opportunity for energy recovery also exists during downhill running situations with this potential energy, also normally dissipated in conventional propulsion systems.
The average level road power requirement for CBD operations in dense traffic is about 1.8 kW/t and about 3 kW/t when moving with a velocity in excess of 5 km/h. This power consumption for a full sized bus results in power demands of 25 kW and 40 kW, respectively. This is well below the maximum power demand which, for a 15 tonne omnibus, is of the order of 150 kW. Braking losses are particularly significant during CBD operations, where up to four stops per kilometre are common. There are also noise and air pollution problems attendant with the use of diesel fuel, such as the production of soot which some health authorities state is carcinogenic.
In transit operations, there is generally a high peak:average power ratio which has customarily led to the specification of diesel engine for such applications, because of this engine's combination of constant compression ratio and low pumping losses at all torque levels. These characteristics of compression ignition engines are in sharp contrast to spark ignition engines wherein power output is controlled by throttling the engine intake fluid, thereby reducing the compression pressure (and hence the maximum combustion pressure and the efficiency of the combustion process) as well as incurring increased pumping losses.
Turning to conventional electric vehicles, a significant problem is the low energy density of standard batteries, such as the lead acid type, along with the relatively high capital cost of suitable power electronic systems for implementing regenerative operations. Furthermore, reduced vehicle performance is experienced as the batteries approach a low state of charge. Traction batteries typically possess an energy density of about 100 kJ/kg at a 3 hour rate of discharge, but only about 50 kJ/kg at a 30 minute rate of discharge. Conventionally battery packs in electric vehicles constitute up to 30% of the vehicle mass.
It is extremely difficult to transfer more than 70% of vehicle kinetic energy back into a battery pack of the above mentioned mass. For example, if a vehicle was braked from 60 km/h for 10 to 12 seconds, the electric machine and electronic system would need to deliver 50 W/kg to the battery at an efficiency of perhaps 80% for the electric machine and 85% for the battery, resulting in an overall efficiency of 68%. When braking from higher speeds the efficiencies are worse. Thus an electric drive is not really suited to stop-start operations. Furthermore, the electric machine has to have a sufficiently high power rating in order to be compatible with normal traffic, which requires peak powers of around 15 kW/t of vehicle mass. For example an AC machine rated at 180 kW for powering an omnibus has large losses when delivering the average level road power of 40 kW.
The prior art is replete with examples of hybrid propulsion systems for vehicles wherein a combustion engine and an electrical machine, operating as a motor, are used as propulsion units. U.S. Pat. No. 5,343,970 (Severinsky) describes a typical hybrid arrangement wherein an AC induction motor drives the vehicle at low speeds or in traffic, whilst an internal combustion engine drives the vehicle in highway cruising. The electric machine is supplied by a bidirectional AC/DC power converter and is operable as a generator to charge storage batteries, during braking or from the engine. Both propulsion units may together drive the vehicle during acceleration or hill climbing situations. The Severinsky arrangement is an example of a "parallel hybrid" system wherein the propulsion units can each provide power via a torque sharing device coupled directly to a vehicle's final drive. The specification also includes a useful review of prior art propulsion systems. U.S. Pat. No. 5,562,566 (Yang) is another example of a hybrid propulsion system of this type.
U.S. Pat. No. 5,318,142 (Bates et al.) is an example of a "series hybrid" wherein only one propulsion unit supplies torque directly to the final drive. A further example is disclosed in U.S. Pat. No. 5,515,937 (Adler et al.), which happens to employ individual motors at each wheel in the final drive. As set out in Severinsky the cost, weight and inefficiency limit the performance of series hybrid type propulsion systems.
There also exist hybrid propulsion systems which employ subsidiary energy storage systems, other than the ubiquitous electric machine and battery combination, to recover energy normally dissipated during braking or downhill running situations for re-use in accelerating and driving the vehicle. U.S. Pat. No. 4,441,573 (Carmen et al.) describes an engine and hydraulic machine in a parallel hybrid arrangement, including an internal combustion engine and a variable displacement hydraulic pump/motor coupled, in one form, by a power splitting planetary gear assembly to the final drive. The hydraulic pump/motor is supplied by a series of high pressure hydraulic accumulators for storing fluid pressure, transferred to them during regenerative pumping or supplied from them during propulsive motoring of the hydraulic machines. A further example of this configuration is disclosed in U.S. Pat. No. 4,813,510 (Lexen) which is designed for vehicles conducting non-transit type start-stop operations, such as CBD buses.
Carmen also observes that flywheels may also be used as a subsidiary storage system either in substitution for, or in combination with, a hydraulic accumulator. There is, however, no discussion of how the latter might be put into practical effect. The '573 specification also contains a useful review of the prior art relating to flywheels and hydraulic accumulators in the field of hybrid propulsion systems.
U.S. Pat. No. 5,492,189 (Kriegler et al.) describes a hybrid propulsion system which includes an internal combustion engine operating in a steady state mode and a driving engine operating in a transient mode, which act on the output shaft of a planetary gear train. The two transient engines may be configured as hydraulic machines or electric machines whereby power flow between the hydraulic or electric machines, and an associated energy storage unit, is controlled by a suitable control unit.
Similarly, U.S. Pat. No. 5,495,912 (Gray, Jr. et al.) describes a hybrid power train including a small internal combustion engine which may be coupled to a continuously variable transmission (CVT) and/or to a hydraulic machine, the torque of which machine is reversible so as to act as either a pump or a motor. Gray indicates that a secondary engine, for example another internal combustion engine, might be clutched to the first I.C. engine to provide additional power for repeated acceleration or ascending long grades when the hydraulic accumulator supplying the hydraulic machine is depleted. In a further example, the second engine comprises another hydraulic machine which is broadly similar to the Kriegler configuration.
Whilst it is apparent from the above discussion that hybrid propulsion systems using a combination of combustion engine and electric machine/battery or a combination of combustion engine and hydraulic machine/ accumulator and/or flywheel are known, the combination of electric machine and hydraulic machine/accumulator is much less often employed in practice. A hydraulic propulsion unit employing a nitrogen oil accumulator operating at 345 bar exhibits an energy density of about 1.5 kJ/kg and allows a relatively high rate of energy transfer, both to and from the accumulator. However, high power levels require large hydraulic machines which are conventionally clutched out of service when not needed for propulsion or retardation.
A hydraulic drive unit employing a hydrostatic pump/motor and accumulator also has an excellent characteristic for regenerative braking as the torque rises with the pressure in the accumulator and hence at zero speed the hydrostatic system, if the accumulator is fully charged, has maximum torque capability. However, the oil nitrogen accumulators need to be relatively large and a pressure ratio between the maximum and minimum pressure of approximately 3:1 means that as the accumulator is drained of its oil, the pressure is reduced to one third, which means that the torque and hence power capability of the hydrostatic machine at speed is considerably less than at its maximum possible operating point.
A large 15 tonne omnibus could easily be fitted with a 355 cc hydraulic machine with a maximum motoring power of 400 kW or a torque of 1950 Nm when directly connected to the final drive. The torque is about 2.5 times the maximum torque of similarly rated diesel engine. If the machine were not clutched out of service when not required, the drag at zero pressure differential and zero oil flow is equivalent to a 9% increase in the drag of the omnibus. This loss is unacceptable for an omnibus with a hybrid electric/hydraulic propulsion system required to operate in transit mode over a medium distance.
A further restriction on the use of accumulators is the high energy loss if the energy is put in adiabatically, with a polytropic index of up to 1.6 for a 100 to 345 bar rapid compression in 15 seconds. Where a hybrid propulsion system relies on a full accumulator charge during low speed high rate acceleration from zero speed, the pressure drop due to cooling of the gas necessitates a small energy addition to accumulators with power levels of the order 3 kW when the vehicle is stationary. This energy addition should be the normal method of utilisation of accumulators in hybrid systems and is of utmost importance in electric powered vehicles. Accordingly the necessary energy addition is likely to be a major factor for their present non-use in electric hybrid vehicles.
The kinetic energy at 80 km/hr, the normal maximum speed for 12 tonne transit omnibuses, is approximately 2.9 MJ. This kinetic energy minus the drag energy has to be stored at rates of approximately 200 kW for the 10 seconds required for a very rapid stop if full regeneration of energy is required. However, the actual acceleration requirements are somewhat less, being spread over 15 to 20 seconds, requiring a very large wheel torque in the speed range up to 30 km/hr.
A flywheel is an ideal source and sink of transient energy since kinetic energy, of the order mentioned above, can be stored in a steel flywheel operating at normal maximum speeds of automotive systems, typically 5000 to 6000 rpm. However, to utilise a flywheel over the speed range of a bus normally requires a continuously variable transmission. Generally, this is difficult to achieve since there is a large speed mismatch between the flywheel and the vehicle, as the flywheel should be at maximum speed when the vehicle is stationary and vice versa. If electric machines are used to produce a continuously variable transmission (CVT), ie. if one electric machine is mechanically coupled to the flywheel, another electric machine is mechanically coupled to the drive shaft and both machines are connected electrically, the motor connected to the drive shaft has to be relatively large to produce the low speed high torque required for an omnibus. An AC machine, such as an induction motor, is typically directly coupled for energising the flywheel. Although the AC machine may be considerably smaller than the main drive motor, induction machines have poor power capability at high speeds. Accordingly a large expensive AC machine is generally required. Such machines have high eddy current and hysteresis losses at high frequency, where most of the kinetic energy is stored.
A further species of hybrid propulsion systems, are those which employ three or more different types of propulsion unit in order to meet highly variable road load conditions, whilst attempting to maximise efficiency. German Patent Application No. 3842632 (MAN Nutzfahrzeuge AG) describes a system including an internal combustion engine and a pair of hydraulic motors coupled to a planetary transmission, along with an electrical machine which is clutched to a flywheel. The electrical machine is operable as a motor supplied from a storage battery and as a generator for charging the battery. The system described relies on the battery and (presumably) a fluid fuel tank for storing energy but does not include any fluid pressure storage means.
The propulsion units described in the MAN specification are connected to a very complicated epicyclic gearbox through which each unit transfers all its power to the final drive. This arrangement has several inherent operational drawbacks, including the requirement for relatively large hydraulic and electric machines in order to deliver sufficient torque, particularly when accelerating the omnibus. In one mode of operation the engine drives a first hydraulic machine as a pump which transmits power (via the interconnecting hydraulic circuit) to the second machine which acts as a motor to drive the tail shaft. There are considerable losses associated with the clutching of the first hydraulic machine between different gears in the gearbox. As engine load levelling at cruise is obtained by charging and discharging the battery, power transfer from the engine is likely to be inefficient because the hydraulic machine transmits from 30% to 50% of the total power.
Thus existing hybrid propulsion systems suffer from a number of drawbacks and disadvantages in vehicle and particularly omnibus operations. The wide variety of prior art attempts to address these problems combined with the relative absence of such systems in volume commercial production is itself indicative of the failure to identify a satisfactory solution.